The present invention relates generally to additive fabrication equipment, and more specifically to adaptations whereby such equipment can be operated under closed-loop feedback control in response to changes in an image of the immediate growth environment. Such adaptations can greatly improve the dimensional tolerance and minimum feature size attainable by such machines.
Laser engineering net shape (LENS) fabrication is an example of a laser-driven additive fabrication technique, wherein a high-energy density laser beam is used to drive localized deposition of material on a surface, and by repeating this process to build up a desired component. Such additive fabrication techniques stand in contrast to traditional fabrication techniques, in which material is removed from an original billet until the desired part remains. LENS fabrication is a promising manufacturing technology, which has the potential to significantly reduce the length of time required to go from initial concept to finished part. Note that the general principles of the present invention also apply to direct fabrication technologies in which the laser is substituted by other sources of controllable, concentrated energy, such as particle beams, plasma beams, and electrical arcs.
In the LENS process, a component is fabricated by focusing a laser beam onto a locus on a growth surface so as to create thereabout a molten pool. The locus is then moved along the growth surface with a speed called the traverse velocity, pulling along with it the molten pool, while a growth material (often a fusible powder, although feed wire has been used) is injected into the molten pool. A portion of the growth material melts and becomes incorporated in the molten pool. As only enough heat is applied to maintain a given volume of liquefied material, the excess growth material is deposited onto the growth surface as the locus moves. The amount and type of material added to each point of the growth surface can be varied, and numerous layers can be grown atop the original layer, leading to additive fabrication of the desired component.
The LENS fabrication approach has proven to be quite flexible. Materials including steel alloys, titanium alloys, and Ni-based superalloys have proven well suited to the technique. In addition, growth involving graded material compositions and highly detailed structural configurations, including buried hollow structures, have been demonstrated.
Lens fabrication takes place at speeds which are practical for a wide variety of high-value components. For example, a laboratory-scale LENS apparatus (laser power xcx9c500-1000 watts continuous) can write a line several hundred microns thick and wide, and composed of steel, at a rate of about 2 meters/minute. The amount of steel added in an hour""s operation is then about 100-150 grams, meaning that even a small LENS apparatus can fabricate components (e.g., precision molds and dies) having a high enough value to justify the cost of the process. The LENS process is also capable of creating objects with unusual compositions, structures, and/or graded compositions which are nearly impossible to fabricate using other techniques.
The LENS process is particularly useful if it can be used to produce components which are essentially ready for use, i.e., in which the dimensional tolerances, surface finish, and global heat treatment are satisfactory for the intended use. The final dimensional and polishing steps are usually the most costly in conventional fabrication. A commonly mentioned candidate is a die or mold, which might require dimensions accurate to 10 microns, and a surface finish of a micron or less.
This level of precision is very difficult, and perhaps impossible, to achieve in the LENS process, in which the molten pool typically has dimensions of several hundred microns. Even small variations in process parameters will introduce errors which are much greater in size than the required tolerances. However, proper control of the operation of a LENS system offers the potential to reduce severalfold the scope of the precision finish machining required, a result greatly desired by end users.
The process parameters of a LENS system, or fabrication system of similar type, are the instantaneous operating parameters of the apparatus. These would include such characteristics as the output power of the laser, the laser power actually directed onto a locus on the growth surface, the rate at which growth material is added to the molten pool surrounding the locus on the growth surface, the path and velocity of the locus about the growth surface, time and amount of z-axis adjustments, rate of flow of cooling gas onto the locus, and so on.
In any given apparatus, some of these process parameters will be separately controllable, and others will be substantially fixed during operation. Those parameters which can be adjusted en passant during an ongoing fabrication process are termed controllable parameters.
Distinguished from the process parameters, which go to the description of the state of the apparatus and its alterations during the growth process, are the intrinsic parameters. Intrinsic parameters describe the environment within which the growth process is actually taking place, and include such information as temperature distribution on the growth surface, size and shape of the molten pool, maximum degree of pool superheating, the trailing thermal gradient (the rate at which temperature returns to ambient along the path taken by the locus), area of specific isotherms, thickness of the growth layer, depth of penetration of growth-related thermal transient, and the like. The common feature is that the intrinsic parameters are affected by the state of the LENS apparatus, but cannot be directly controllable parameters. E.g., the temperature at the locus cannot be set as a controllable parameter, but represents a balance between several process parameters and material parameters, including laser power, velocity of locus, material addition rate, thermal conductivity of the various materials near the locus, and so on.
The limitations of open-loop LENS processes can easily be observed. By open-loop, we mean setting and maintaining the process parameters without real-time feedback concerning the intrinsic parameters, i.e., concerning what is actually happening at the growth surface. Simply providing feedback control, e.g., to keep the laser power constant at the laser output, is still considered open-loop operation. Only when intrinsic parameters are measured and used to provide feedback information is the control system of the closed loop type.
Typical open-loop LENS results in tool steel and similar materials show dimensional variations parallel to the growth plane as small as 50 microns, and surface finish as good as 10-20 microns RMS, both reasonably encouraging values. However, variations in the height of a built-up component (e.g., a component a centimeter tall comprising perhaps 30-40 deposited layers) are more typically 2-300 microns in size. Such open-loop LENS techniques are simply not suited to most high-precision net-shape fabrication requirements, in that extensive finishing steps of conventional polishing and/or machining are required before a component is ready for use.
The examples of open-loop LENS fabrication described above suggest that there are sufficient reasons to seek a real-time feedback-based closed-loop control system for such fabrication methods. Among these are the need to reduce dimensional variations and surface finish of components, so that they can be used for their intended purpose with a minimum of (ideally with no) additional machining or finishing steps. Similarly, it can be beneficial during growth of a component with compositional gradients to control the process environmental parameters, i.e., the conditions at the point of growth, rather than the gross process parameters of the LENS apparatus. These factors can be degraded by poor control over the immediate thermal environment during the deposition process, and hence respond well to closed-loop control.
The present invention is of an imaging-based feedback-controlled direct laser fabrication system for additive fabrication of net-shape or near-net-shape components. Such a system includes a conventional LENS apparatus (or similar system), which is operated by a feedback system. An imaging sensor directed at the molten pool on the growth surface provides feedback on the actual growth conditions. This type of additive fabrication system produces components with dimensional tolerances and surface quality superior to conventional LENS systems.